Motor vehicles come equipped with an electrical system that is supplied with voltage by an electrical machine operated as a generator, for example, an externally excited synchronous motor. The vehicle system voltage can thereby be regulated by controlling an excitation current of the electrical machine. The electrical machine is generally connected via a rectifier to the vehicle electrical system and, together with the same, forms a generator unit. Faults, such as short circuits, for example, can occur in such generator units and should be detected whenever possible.
A method for detecting a fault in a generator unit is described in German Patent Application No. DE 10 2015 211 933. It discusses an electrical machine that includes a rotor winding and a stator winding and a rectifier connected thereto, via which the electrical machine is connected to an electrical system of a motor vehicle; a voltage of the electrical system being regulated to a setpoint value and a characteristic curve of the excitation current being monitored as a function of an excitation current traversing the rotor winding of the electrical machine; and a fault in the generator unit being inferred upon detection of an oscillating characteristic of the excitation current; a magnitude of the oscillation exceeding a threshold value.